1. Field of the Invention
The invention relates to the field of measurement-while-drilling systems in a downhole environment. Specifically, the invention is a method of measuring azimuthal resistivity values taken in a measurement-while-drilling device in a downhole environment.
2. Description of the Related Art
Measurement devices comprised of multiple transmitter and receiver arrays have been employed in prior borehole induction logging. The primary parameter of interest is the resistivity of the virgin or uninvaded formation, from which the hydrocarbon saturation of the formation is determined. Symmetric pairs of transmitters and receivers have been employed to minimize or “cancel” the effects on the resistivity measurements of rapidly changing borehole parameters such as borehole diameter, eccentricity and rugosity.
U.S. Pat. No. 4,899,112 to Clark et al. discloses a well logging technique in which electromagnetic propagation waves are used to measure formation resistivity at different radial depths of investigation. In addition, Clark '112 teaches methods for determining the existence, location and properties of beds and caves, and also teaches a method for determining changes in the size of the borehole. These measurements are based upon the observation that phase and amplitude of apparent resistivity measurements, made at a given transmitter frequency and a given transmitter-receiver spacing, exhibit different depths of investigation. Multiple transmitter-receiver spacings have also been employed in prior art to obtain measurements into the formation of varying radial depths of investigation. Combining such measurements tends to minimize borehole effects as well as to yield information concerning the radial extent of the invasion of drilling fluid into the virgin formation. Invasion measurements can be related to the permeability of the formation which, in turn, is related to the producibility of fluids contained within the formation rock matrix. No attempts have been made in the prior art to obtain quantitative measures of physical characteristics of the borehole in conjunction with measures of electromagnetic properties of the formation. Multiple transmitter frequencies have also been applied in the prior art to enhance and separate electromagnetic properties of the formation such as resistivity and dielectric constant, obtaining varying effective radial depths of investigation and to a lesser extent to minimize borehole effects. Once again, contributions from the borehole effects have not been quantified and related to the physical condition of the borehole.
U.S. Pat. No. 6,288,548, issued to Thompson et al., the contents of which are incorporated herein by reference, discloses an apparatus for resistivity measurements in a measurement-while-drilling device. The apparatus comprises a transmitting member for generating an interrogating electromagnetic field for passage through the borehole and surrounding formation. A measurement tubular is also provided which comprises a central bore which communicates with a central bore of the drillstring. Said measurement tubular couples in the drillstring to locate the measurement sonde in a particular position, and to permit interrogation of the borehole and surrounding formation with the interrogating electromagnetic field. A means is provided for securing the measurement sonde in a particular location within the central bore of the measurement tubular. The measurement-while-drilling apparatus is operable in at least a transmission mode of operation and a reception mode of operation, which preferably occur simultaneously. During transmission operations, the interrogating electromagnetic field is generated by the measurement sonde and radiated outward from the measurement sonde and through the measurement tubular into the borehole and surrounding formation. During reception operations, the interrogating electromagnetic field passes from the borehole and surrounding formation through the measurement tubular for detection by the receiving member.
The invention of U.S. Pat. No. 5,892,361, issued to Meyer, Jr. et al. discloses a method measuring electromagnetic parameters by detecting phase and amplitude of induced signals at different receivers. Transmitters are activated sequentially at a first frequency. The phase and amplitude of the induced electromagnetic signals within the receivers are measured, yielding two measurements of amplitude and two measurements of phase shift for each transmitter activation for a total of sixteen measurements. The procedure is then repeated at a second transmitter frequency yielding an additional two measurements of amplitude and two measurements of phase shift for each transmitter activation for an additional sixteen measurements. An apparent resistivity measurement is calculated from each of these thirty-two uncorrected “raw” measurements. Each apparent resistivity calculation, being uncorrected as previously mentioned, is greatly affected by the borehole and the near borehole environs. These raw measurements and corresponding apparent resistivity calculations are used, therefore, to determine borehole characteristics such as borehole diameter, rugosity and eccentricity as well as for correcting apparent resistivity measurements for these borehole effects. Stated another way, the invention not only provides formation resistivity measurements corrected for perturbing effects of the borehole, but also provides a method for quantifying these corrections, thereby providing useful information on the physical properties of the wellbore. These wellbore properties, in turn, can be related to such parameters as mechanical properties of the rock matrix, shallow invasion profiles, and the effectiveness of the drilling program.
The borehole instrument portion of Meyer Jr., '361 comprises an elongated mandrel such as a drill collar and a measurement-while-drilling (MWD) embodiment. Two receivers comprising coils of one or more turns are wrapped around the outside diameter of the drill collar and spaced longitudinally along the center of the drill collar. Four transmitters comprising coils of one or more turns are wrapped around the outside diameter of the drill collar and are spaced symmetrically and on either side of the midpoint between the two receiver coils. However, the method of Meyer Jr. '361 does not provide a resolution of resistivity values along the circumference of the drill tool.
U.S. patent application Ser. No. 10/262,548 of Fanini, et al, discloses an induction logging tool that includes transverse coils. With the device disclosed therein, it is possible to make measurements of resisitivities of earth formations that are directionally sensitive. There are several prior art patent applications and that discuss the interpretation of data acquired with such a multi-component tool. One particular application of such multicomponent resistivity tools is determination of a distance to a bed boundary in a substantially horizontal borehole. The hardware of such multicomponent tools is relatively complex, and there may be problems ensuring the relative calibration of the different components.
There is a need for a quick efficient method of resolving an azimuthal resistivity or instantaneous value of resistivities measurements. The present invention described herein meets that need.